The TYPO3 base system is called the core. The functionality of the
core can be expanded, using extensions. A small, selected number of
extensions (the system extensions) are being distributed as part of
the TYPO3 core. The core and its system extensions are being developed
by a relatively small team (40-50 people), consisting of experienced
and skilled developers. All code being submitted to the core is
reviewed for quality by other core team members.

Currently there are more than 5500 extensions available in the TYPO3
Extension Repository (TER), written by some 2000 individual
programmers. Since everybody can submit extensions to the TER, the
code quality varies greatly. Some extensions show a very high level of
code quality, while others have been written by amateurs. Most of the
known security issues in TYPO3 have been found in these extensions,
which are not part of the core system.